Upwelling flow dynamics in long canyons at low Rossby number

Abstract

[1] Submarine canyons, topographic features incising the continental slope, vary in both shape and size. The dynamics of short canyons have been observed and described in the field, in the laboratory, and with numerical simulations. Flow within long canyons, such as Juan de Fuca canyon, located between Vancouver Island and Washington State in the Pacific Northwest, is less well understood. Physical models of both long and short canyons have been constructed to understand the upwelling dynamics in long canyons and how upwelling changes, as compared with the dynamics of short canyons, at low Rossby number. Stratification and rotation, both important parameters in determining the dynamics in canyons, can be controlled and scaled accordingly for replication of oceanic conditions. The physical model is spun up to an initial rotation rate, and the flow is forced by increasing the rotation rate over the equivalent of several days. Flow visualization is used to determine the strength and location of upwelling, the strength and mechanisms generating vorticity, as well as the differences between the flow within the long and short canyons. The pattern of upwelling between the two canyons is significantly different in the horizontal with upwelling occurring through the canyon head in the short canyon and upwelling occurring close to the mouth along the downstream rim in the long canyon. At high Rossby number, upwelling is similar in both the long and short canyon and is driven by advection. However, as Rossby number decreases, the flow in the long canyon is more strongly affected by the strong convergence of the isobaths near the canyon than by advection alone.

A review of the role of submarine canyons in deep-ocean exchange with the shelf

“…Downwelling patterns are seen over Astoria Canyon during upwelling wind relaxations (Hickey, 1997). Time-dependent flow patterns appear to dominate over advection flow patterns for weak flows in the laboratory (Waterhouse et al, 2009). …”

“…The advection dominated response is strongly dependent on the canyon topography and flow strength (Allen, 2004). For weaker flows it can be greatly enhanced if strongly convergent isobaths occur over the canyon (Allen, 2000;Waterhouse et al, 2009). The final relaxation phase sees strong, generally cyclonic flow within the canyon (Hickey, 1997).…”

“…However, cyclonic vorticity is also generated in the slope flow due to friction against the slope (Perenne et al, 2001b). There is some evidence that this latter vorticity may be less important than stretching (Flexas et al, 2008;Waterhouse et al, 2009). These vorticity patterns, with strong cyclonic vorticity (under both upwelling and downwelling) are for near uniform shelf flows.…”

“…This convergence leads to upwelling particularly in long canyons that closely approach the coast (Allen, 2000) even if these canyons are very wide and trough-like (Yuan, 2002). Upwelling tends to occur at the converged isobaths whether they occur near the head of the canyon (Yuan, 2002) or near the mouth (Waterhouse et al, 2009). Some of the clearest observations of enhanced upwelling near isobath convergence is seen at the head of Bio Bio Canyon (Sobarzo et al, 2001).…”

Dynamics of advection‐driven upwelling over a shelf break submarine canyon

“…Vorticity in this flow can be generated by both flow separation [ Pérenne et al , 2001] and by stretching [ Hickey , 1997]. Recent laboratory [ Waterhouse et al , 2009] and observational studies [ Flexas et al , 2008] suggest that the latter probably dominates.…”